[bug-fix] Optimization plots freeze up between updates

[alvinccli]

During optimizations, the optimization plots (showing the target, dose, error, etc) is only updated once per iteration (i.e. every couple seconds or tens of seconds) and the plot is not responsive between the updates.

In other words, the plot window would hang for most of the time.

Solution would be moving the plotting functions to another thread or process such that the plot remains responsive.